US intelligence agencies did not detect the Syrian regime readying a massive chemical weapons attack in the days ahead of the strike, only piecing together what had happened after the fact, US officials say.

One of the key pieces of intelligence that secretary of state John Kerry
later used to link the attack to the Syrian government - intercepts of communications telling Syrian military units to prepare for the strikes - was in the hands of US intelligence agencies but had not yet been "processed," according to senior US officials.

That explains why the White House didn't warn either the regime or the rebels who might be targeted as it had done when detecting previous preparations for chemical strikes.

"We know that for three days before the attack the Syrian regime's chemical weapons personnel were on the ground in the area making preparations," Kerry said as he presented the evidence in a state department speech last week.

"We know that the Syrian regime elements were told to prepare for the attack by putting on gas masks and taking precautions associated with chemical weapons."

But the Obama administration only uncovered the evidence after Syrians started posting reports of the strike from the scene of the attack, leading US spies and analysts to focus on satellite and other evidence showing a Syrian chemical weapons unit was preparing chemical munitions before the strike, according to two current US officials and two former senior intelligence officials.

They spoke on condition of anonymity because they were not authorised to discuss the intelligence publicly.

The spokesman for the director of national intelligence confirmed that US intelligence did not detect the massive chemical weapons attack beforehand.

"Let's be clear, the United States did not watch, in real time, as this horrible attack took place," Shawn Turner said in a statement to The Associated Press on Wednesday.

"The intelligence community was able to gather and analyse information after the fact and determine that elements of the Assad regime had in fact taken steps to prepare prior to using chemical weapons," Turner said.

Turner offered no reason for the delay in processing the intelligence, but current and former intelligence officials said analysts were stretched too thin with the multiple streams of intelligence coming out of multiple conflict zones, from Syria to Libya to Yemen.

In December, US intelligence detected Syria's military was readying chemical weapons for use, and President Barack Obama warned the Syrian government publicly that such use was "totally unacceptable" and that the country's leaders would be held accountable.

The White House is now asking Congress to approve a punitive strike against the regime of Syrian President Bashar Assad, which the administration blames for an August 21 chemical weapons attack in a rebel-held suburb of Damascus.

The administration says 1,429 died in the attack. Casualty estimates by other groups are far lower.

Kerry and other officials are laying out the intelligence in open and closed sessions with lawmakers, explaining why the US intelligence community last week issued a "high confidence" report implicating the Syrian regime - a conclusion echoed by British and French intelligence in similar reports made public since the attack.

Senior administration officials explained last week that the US intelligence community had reconstructed a picture of the attack, from satellite and signals intercepts that indicated to them that troops from Syria's military unit that handles chemical weapons, the Syrian Scientific Studies and Research Center, were readying such weapons.

That conclusion was backed up, however, by a carefully written sentence that indicated the intelligence was somewhat circumstantial: "Syrian chemical weapons personnel were operating... near an area that the regime uses to mix chemical weapons, including sarin."

The report says US intelligence intercepted communications after the attack by a "senior official intimately familiar with the offensive" who confirmed that chemical weapons were used by the Syrian government, and was concerned that the UN inspectors might find evidence of the attack.

The report also says the US has intelligence that Syrian chemical weapons personnel were directed to "cease operations" on the afternoon of August 21, several hours after the attack.

The US officials briefed on the intelligence say such intercepts were in hand but waiting to be processed among hours of intercepted military communications.

The CIA and the defense intelligence agency have dozens officers on the ground in countries neighboring Syria, relying on a network of rebels and local agents to provide human intelligence on the goings on of both the regime and its opponents.

The Pentagon also has satellites focused on the area, capturing images of the regime and rebel maneuvers, while various types of airborne platforms collect electronic transmissions such as military radio traffic or cellphone calls.